Positive working photoresists are known in the art. They generally comprise film-forming polymeric resin binder containing a photoactive compound. The resin binders most frequently used are made from phenol-formaldehyde condensation products, such as Novolak for example. Positive resists are prepared using the novolak type resin by incorporating therein a photoactive compound, for instance, one of the group of the 4- or 5-substituted diazonaphthoquinone (1,2)-diazide sulfonic acid esters. The alkali soluble novolak resins, when mixed with a photoactive acid-generating compound, become insoluble in aqueous alkaline solutions. However, areas exposed to actinic radiation become soluble to these alkaline solutions, called developers, as acidic groups are generated.
Positive resists are most often used as masks to protect substrates from chemical etchants in the manufacture of semiconductors. In such manufacture, the photoresist is coated onto the surface of a semiconductor substrate, and then imaged and developed to remove areas subjected to actinic radiation. The resist image remaining on the surface of the substrate is usually employed as a protective mask to facilitate the selective etching of the exposed portions of the substrate thereby defining a circuit pattern.
Etching of the substrate may be conventionally carried out by chemical treatment, or by dry etching, e.g. reactive ion etching with chemically active gas ions, such as fluorocarbon species for example, formed by glow discharge. Additionally, modern techniques for processing semiconductors may call for plasma and sputter etching, ion beam implantation, and the like.
One of the problems associated with these techniques is that many resist materials cannot withstand the process severity and are eroded along with the substrate, or flow due to reaction with the gaseous ions and the rather high temperatures often encountered on the semiconductor substrate (typically above 200.degree. C.) resulting in loss of pattern resolution. For example, many novolak type phenol formaldehyde resins begin to flow at temperatures around 120.degree. C. and erode when struck by the gas stream generated during reactive ion etching.
When photoresist compositions are employed as dielectric layers, not only thermal stability is essential, but the resist must also maintain good dielectric and mechanical properties.
Over the years, polyamic acid resins produced by condensation of an aromatic dianhydride and an aromatic diamine have received widespread attention as polymeric binders for photoresist compositions because they are readily converted by heat to thermally stable polyimides with very balanced mechanical and dielectric properties. However, the use of polyamic acids has been restricted for the most part to negative working photoresist compositions because of their high solubility in alkaline solutions.
Several problems associated with the use of negative working polyimide photoresists for imageable dielectric layers could be overcome if positive working photoresists were used. In the first place, dielectric applications commonly require that holes be patterned in the existing coating. Hole patterning is most effectively accomplished through the use of a positive resist, in which the exposed areas are removed. In addition, a slight side slope is desired in order to achieve effective metal contacts. Positive resist naturally achieve the necessary sloping sidewalls because the top of the film receives a higher exposure than the bottom of the film, and is therefore slightly more soluble. A third advantage of positive resist over negative resist is due to the fact that the spec of dust on photomask is not so critical. Higher manufacture yield due to lower defects can be improved with positive working polyimide. Positive working resists are also distinguished, as compared to negative resist, by high resolution due to less image swelling, the ability to utilize aqueous developers, which is of importance ecologically and economically, and by the fact that the presence of oxygen has no effect on the exposure time.
Attempts have been made to prepare positive working polyamic acid based compositions using diazoquinone-(1,2)-diazide-sulfonic acid esters such as disclosed in U.S. Pat. No. 4,880,722, among others. Thus, attempts have been made in the past to render the polyamic acid resin insoluble in an alkaline developer by admixture with a orthonaphthoquinone diazide compound. It was believed that sufficient quantities of the diazide compound would render the unexposed areas of the photoresist composition completely insoluble in aqueous alkaline developing solution because of the hydrophobicity and insolubility of the diazides themselves before photolysis or photoimage development. By use of this different solubility, it was believed that sharp distinction between imagewise exposed and unexposed areas during development would occur and thus ensure that only exposed areas would be dissolved in the developer; while unexposed areas would remain insoluble and unaffected in the developer. However, such attempts have had only limited success in that polyamic acid based photoresist systems exhibit such high dissolution rate in conventional commercially available alkaline solutions such as tetramethyl ammonium hydroxide, that adequate control over the process to obtain high resolution can only be achieved with weak alkaline developers such as dilute (0.5%) diethylethanolamine, for example.
Attempts to decrease the dissolution rate of the polyamic acid photoresist precursor, by increasing the concentration of the photoactive compound in the photoresist, e.g. up to about 50% by weight, increase the optical density of the photoresist to such a high extent, that full penetration of the film thickness by a radiation source is unattainable for all practical purposes.
In U.S. Pat. No. 4,880,722 (Moreau, et al.), it is disclosed that the dissolution rate in alkaline developers of image-wise exposed photoresist systems based on diazoquinone sensitized polyamic acid is reduced to prepare relief images of fine line resolution by reducing the acidity of the polyamic acid prior to exposure. This reduction in acidity is achieved by pre-baking the coating to partially imidize the polyamic acid to a level of 20% imidization; or partial neutralization with basic organic reagents; or the use of blends of the polyamic acid with its ester derivatives or with copolymers of the acid and its ester. However, pre-baking to achieve partial imidization above 100.degree. C., as for example at even 120.degree. C., causes loss of photosensitivity due to degradation of the diazoquinone photosensitizers. Furthermore, acidity reduction through employment of basic organic reagents, therein disclosed, tends to promote corrosion of the conductors found in integrated circuits. Blends of polyamic acid with its ester derivatives tend to cause phase separation during pre-baking process.
In addition, resist formulations which contain polyamic acids and diazoquinone compounds have limited storage life, since diazoquinone compounds tend to decompose in the presence of acid.
U.S. Pat. No. 4,863,828 (Kawabe et al.) discloses a positive working photoresist composition, which comprises a light sensitive substance of 1,2-naphthoquinonediazide-4-and/or -5-sulfonate of 2,3,4,3',4',5'-hexahydroxybenzophenone and an alkali soluble novolak resin dissolved in ethyl lactate or methyl lactate. This reference also discloses that the composition may further contain a polyhydroxy compound for accelerating dissolution of the composition into a developer, in a preferable amount of 0.2 to 5% by weight based on the solid contents of the composition.
U.S. Pat. No. 4,738,915 (Komine et al.) discloses positive-working photoresist compositions comprising a novolac resin and an ester compound between 2,3,4-trihydroxybenzophenone and naphthoquinone-1,2-diazido-5-sulfonic acid. In addition, the composition contains a specified amount of 2,3,4-trihydroxybenzophenone in a specified amount relative to the ester compounds as part of the photosensitive component which may be a reaction product obtained by the esterification reaction for the synthesis of the ester compounds containing unesterified 2,3,4-trihydroxybenzophenone.
U.S. Pat. Nos. 4,626,492 and 4,650,745 (Eilbeck) disclose a composition and a method, respectively, pertaining to a positive resist which is claimed to demonstrate improved photospeed and rate of development. The resist composition contains a solvent and select proportions of a novolac resin, a naphthoquinone diazide sensitizer, a dye which absorbs light and an effective proportion of a trihydroxybenzophenone compound.
U.S. Pat. No. 4,009,033 (Bakos et al.) discloses a positive photoresist which is claimed to have increased sensitivity to light, and which is formed by the addition of an acidic compound to a 1,2-quinone-diazido-sulfonic acid ester sensitizer.
In contrast to the present invention, as it will be explained in detail hereinbelow, none of the above references discloses, suggest or implies the combination of an alkali insoluble resin of the polyimide or polyimide-precursor type with an acid generator and an additive compound, used to enhance solubility, in an amount of 25-50 parts (preferably 30-40 parts) of said compound per 100 parts of resin.